Production of castings



United States Patent f 3,489,202 PRODUCTION OF 'CASTINGS Percy Ronald Taylor, Wrsxham, Wales, assignor to Monsanto Chemicals Limited, London, England, a British company No Drawing. Filed Apr. 6, 1967, Ser. No. 628,845 Claims priority, application Great Britain, Apr. 12, 1966, 15,908/ 66 Int. Cl. B22c N12 US. Cl. 164-138 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the production of a metal casting which comprises the steps of forming a shell mould on a disposable pattern using a particulate ceramic refractory material such as silica, a binding agent such as a condensed ethyl silicate, and a particulate fusible material such as ferro-aluminium. The shell mould is then removed from the pattern and fired and the casting material is then poured into the mould so formed. The above-described fusible material is characterized by having a melting point above the temperature at which the shell is fired but which melts when the molten metal is cast into the mould.

This invention relates to a process for the production of metal castings, in particular to a process of metal casting employing ceramic shell moulds, and to the moulds employed in the process.

A problem which has for a long time hindered the utilisation of ceramic shell moulds for the manufacture of castings from certain types of ferrous metals, for example plain carbon steels, low-alloy steels and many ferritic and martensitic stainless steels, stems from the fact that such metals are oxidised at high temperatures. Under normal conditions of operation, the relatively porous structure of a ceramic shell mould can permit oxidation of the metal adjacent to the shell wall to occur. The nature of the resulting changes in the metal surface vary according to the conditions and the particular metal concerned, but they can be such that the appearance and general quality of the castings are adversely affected.

Various proposals have been made for dealing with this problem, for example that the ceramic shell mould should be located in a non-oxidising atmosphere or a vacuum during the pouring and cooling of the metal or that the mould should be embedded in carbon to effect absorpti n of oxygen from the vicinity of the mould, thus preventing its access to the metal. The present invention is directed to a novel solution of the same problem by providing means whereby the shell is sealed or its porosity is substantially reduced after the metal has been cast but before any significant cooling and ingress of air through the shell can occur.

The process of the present invention is one f r the production of a metal casting, which comprises forming a shell mould on a disposable pattern using a particulate ceramic refractory material, a binding agent and a particulate fusible material, removing the pattern, firing the mould, and casting metal into the mould so formed, the fusible material being one having a melting point above the temperature at which the shell is fired but which melts when molten metal is cast into the mould.

It is preferred that the fusible material should be one having at its fusion temperature a significant affinity for oxygen. Ferro-aluminium and similar alloys are especially suitable.

The practice in the production of conventional ceramic shell moulds is to build up a shell of suflicient thickness by applying to the pattern a succession of coats of particulate refractory material. The first coat is applied as a 3,489,202 Patented Jan. 13, 1970 ICC slurry to the refractory material in a liquid binding agent, usually a silica sol or a hydrolysed silicate ester solution, which is then gelled or set. This is followed by a coating of particles of a refractory material applied as a stucco to the gelled first coat while the latter is still damp. Slurry coatings and stucco are then applied alternately until a shell of the required thickness has been built up. The thickness of each slurry coating is determined largely by the viscosity of the slurry, thicknesses of from 0.2 mm. to 1 mm. being usual. The assembly is then dried, the pattern is removed, for example by melting or dissolving out, and the resulting shell is fired to harden the mould, usually at a temperature in the range of 950 C. to 1100 C., although higher or lower temperatures, for example from 800 C. to 1300 C. can be employed. Firing times normally vary from a minimum of about 15 to 20 minutes to a maximum of about 2 hours, depending on the size and thickness of the mould. A period of about 1 hour is typical.

Each slurry coating and its associated stucco is commonly referred to as an investment layer, the first such layer being termed the primary-investment layer, and the second and subsequent layers, secondary-investment layers.

In the usual manner of operating the process of the present invention, the fusible particulate material is admixed with or replaces the refractory material in one or more of the coatings applied to the pattern in building up the shell. Most conveniently, the fusible material is used as or is included in a stucco coating. A preferred feature is that the fusible material be present as the stucco in the second of the secondary-investment layers or in one or more subsequent layers, but not in the primary-investment or the first of the secondary-investment layers.

Metals having a range of melting points can be cast according to the process of the invention, most of those employed in practice having melting points not exceeding 1750 C.

A further aspect of the present invention is a fired ceramic shell mould having included in the fabric of the mould a fusible material in particulate form having a melting point not exceeding 1800 C. Taking 1000 C. as a typical temperature at which the shell mould is fired, this means that the melting point of the fusible material will usually be within the range 10001800 C. The present process is particularly valuable for the production of castings from steels having melting points of the order of 1450-1550 C., and where the shell mould is required for the production of such castings, the fusible material employed is generally one having a melting point within the range 1000-1600 C., and preferably one having a melting point within the range 1000- 0 C. In a shell mould having the conventional layered structure, the fusible particulate material is preferably distributed substantially uniformly throughout, or forms, the stucco or slurry coating (usually the former) in one or more such layers, more especially a layer other than the primary-investment or first secondary-investment layer.

Suitable fusible materials which can be used include for example certain of the alloys of iron and aluminium, i.e. ferro-aluminiums and of iron silicon and zirconium, i.e. the zirconium-ferro-silicons. The preferred fusible particulate materials are ferro-aluminiums containing from 25 to 50%, especially from 30 to 40% by weight of aluminium. These ferro-aluminiums have melting points of about 1300 C. Examples of suitable zirconium-ferrosilicons are the alloys having the compositions:

zirconium 12-15 silicon 39-43%; balance, apart from incidental impurities, iron;

zirconium 35-45%; silicon 47-52%; balance, apart from incidental impurities, iron;

with melting points in the range 1260-1343 C.

The refractory materials generally available may be used to produce the bulk of the mould, for example, Molochite, silica, zircon and alumina. For use in a slurry coating, it is usual for the refractory to be in a state of sub-division such that none is retained on a 200 British Standard Sieve (B.S.S.) mesh and preferably that it should contain a substantial proportion of particles that are fine enough to pass through a 300 mesh British Standard Sieve (B.S.S.).

For use as a stucco coating, a somewhat coarser refractory material is preferable; this, for example, can usefully contain more than 50% of particles having sizes in the range 40 to 100 B.S.S. mesh, although finer material may also be present. The fusible material used in the process of the present invention should preferably have a particle size range comparable with that of the refractory with which it is mixed or which it replaces.

While, as indicated above, the fusible particulate material may be used in admixture with a refractory material, better results are, in general, obtained where it is used undiluted. A mixture of a particulate refractory material and a particulate fusible material for use in the present invention should normally contain not more than 50% by weight of the former, preferably not more than 25 by weight, and more preferably not more than by weight.

The binding agent used in the production of a shell mould in the process of the invention can conveniently be a silicate ester such as for instance ethyl silicate, for example the condensed ethyl silicate sold as Silester OS, or a silica sol having a concentration of between 20 and 40 percent by weight of silica, such as for example those sold as Syton 2X and Syton P (Silester and Syton are registered trade marks).

While the process herein described can be used for the production of castings of high-chromium steels, its particular advantage lies in the fact that it permits satisfactory casting of plain low-carbon steels, for example B.S. 1617A and CLA9; of low-alloy steels, for example those of the Fortiweld variety, and type EN36C; and high-carbon, high-alloy tool steels generally containing around 12 percent chromium and other alloys which have poor hightemperature scaling resistance.

The invention is illustrated by the following example.

EXAMPLE Two wax assemblies were prepared in the form of one inch square sprues ten inches long and with miscellaneous wax patterns attached.

One wax assembly was used to build up a ceramic shell mould in the conventional way using a primary coating of zircon flour bonded with colloidal silica sol and five secondary coatings of Molochite bonded with hydrolysed ethyl silicate. Molochite material was used as a stucco on all slurry coatings.

The other wax assembly was used to make a ceramic shell mould, the only difference in technique being that ferro-aluminium containing 3540 percent by weight of aluminium, the particles of which varied in size from 40 mesh to dust, was used as stucco in the second secondaryinvestment layer.

After four layers had been applied to the assemblies, the latter were joined together by a wax-pattern of a pouring-cup, and two further coatings of slurry and stucco were applied. The assemblies were dried, subjected to a temperature of 150 C. to melt out the wax, and then fired at a temperature of 1000" C. for one hour.

Using the moulds thus produced, castings were made from a steel having the following analysis: C, 0.15%; Mn, 0.50%; Si, 0.60%; Ni, 1.56%; Cr, 1.49%; S, 0.016%; P, 0.020%; Fe, percent balance, the metal being poured at a temperature of 1560 C. The cast surfaces of the patterns and spun; of the unmodified part of the mould were found to be severely disfigured, as is usual with this and similar types of alloy. On the other hand, the surfaces of the casting made according to the invention were excellent.

What is claimed is:

1. A process for the production of ametal casting which comprises the steps of (1) forming a shell mould on a disposable pattern by applying alternative coatings of (a) a slurry of a particulate ceramic refractory material and a binding agent, and (b) a stucco of a particulate ceramic refractory material per se thereto, said shell mould containing, in addition to items (a) and (b), a particulate fusible material which is admixed with or replaces such refractory material in one or more of said coatings applied to said pattern in building up said shell to a desired thickness; (2) separating said disposable pattern from said shell mould; (3) heating said shell mould for a sufficient period of time and at a temperature in order to substantially harden said shell mould; and (4) casting metal into the hardened shell mould; said particulate fusible material being characterized by having a melting point above the temperature at which the shell mould is heated, but which melts when molten metal is cast into said shell mould, and with the proviso that one coating of item (a) plus one coating of item (b) constitutes a layer, and the particulate fusible material constitutes or is part of the stucco coating of one or more of the layers applied to said disposable pattern starting with the third layer.

2. The process as set forth in claim 1 wherein the stucco coating of one or more of such layers is a mixture of said fusible material and said refractory material, said refractory material being less than 25% by weight of the total weight of said mixture.

3. The process as set forth in claim 2 wherein the fusible material is a ferro-aluminum containing from about 25 to about 50% by weight of aluminum.

4. A process as set forth in claim 3 wherein the metal that is cast is selected from the group consisting of a plain low-carbon steel, a low-alloy steel, a high-carbon high-alloy tool steel, a ferntic stainless steel and a martensitic stainless steel.

5. A fired ceramic refractory shell mould having included in the fabric of such mould a particulate fusible material having a melting point of less than 1800 C., said shell mould consisting of a multi-layered structure in which the particulate fusible material is distributed substantially uniformly throughout, or forms a part of one or more separate layers other than the first two layers which were applied to the disposable pattern.

6. The shell mould as set forth in claim 5 wherein the particulate fusible material has a melting point within the range of from about 1000 to about 1600 C.

7. The shell mould as set forth in claim 6 wherein the particulate fusible material is a ferro-aluminurn.

References Cited UNITED STATES PATENTS 1,566,420 12/1925 Pacz 106-38. 9 2,345,211 3/1944 Neiman 106-383 2,811,760 11/1957 Shaw l0638.3 3,144,690 8/1964 Buckingham 106-38.9 3,158,912 12/1964 Schweikert 164-25 3,326,269 6/1967 Schneider 106-389 3,367,393 2/1964 Lenahan et a1. 16441 X I. SPENCER OVERHOLSER, Primary Examiner I. S. BROWN, Assistant Examiner US. Cl. X.R. 16425, 361 

